1. Field of the Invention
This invention relates to a method for the manufacture of aluminum oxynitride, and more particularly to a method for the manufacture of aluminum oxynitride refractory using aluminum dross as starting material for yielding the desired nitride.
2. Description of the Prior Art
It has been known heretofore to obtain aluminum oxynitride represented by the chemical formula, mAl.sub.2 O.sub.3.nAlN by heating to sinter a mixture of micronized alumina with powdered aluminum nitride. The aluminum oxynitride so produced exhibits satisfactory resistance to fire and heat and possesses outstanding corrosion resistance in molten metal and, therefore, is expected to find extensive utility as a refractory material particularly for use in a reducing atmosphere.
Aluminum oxynitride can be obtained, as described above, by heating under sintering conditions a mixture of alumina powder and aluminum nitride powder. When this method is used, the resultant refractory is inevitably expensive. This is because this specific sintering method makes it necessary to use, as raw material, an alumina which has been adjusted extremely finely to a particle diameter of the order of not more than several microns and, consequently, is relatively expensive when used as a starting material for aluminum nitride powder.
Therefore, other methods have been proposed which use, as a starting material for production of aluminum oxynitride refractory, alumina obtained by Bayer Process which is mass produced commercially and is available at a low price. In the specification of Japanese Patent Publication No. SHO 53(1978)-14,247, for example, a method is disclosed which effects incorporation of nitrogen in electrically fused alumina by taking alumina of Bayer Process and electrically fusing this alumina in an atmosphere of nitrogen gas. By this method, however, a refractory having a high nitrogen content cannot be obtained because incorporation of an adequate amount of nitrogen in the alumina is difficult to attain.
In the specification of Japanese Patent Publication No. SHO 52(1977)-50,040, a method for the manufacture of a nitrogen-containing alumina refractory is disclosed which comprises mixing alumina obtained by the Bayer Process with metallic aluminum powder and electrically fusing the resultant mixture of a nitrogen-containing atmosphere (inclusive of natural atmosphere). The method disclosed in this specification consists in causing the metallic aluminum powder incorporated in advance in the alumina to be converted, during the course of electric fusion of the alumina, by a nitriding reaction into aluminum nitride and effecting incorporation of the aluminum nitride into fused alumina. As compared with the method of the aforementioned specification of Japanese Patent Publication No. SHO 53(1978)-14,247, this method is capable of producing a refractory of a notably high nitrogen content. This method nevertheless has the following disadvantages.
To be more specific, the method disclosed in the specification of Japanese Patent Publication No. SHO 52(1977)-50,040 comprises a linear mixing of alumina from the Bayer Process with metallic aluminum powder and electrically fusing the resultant mixture in a nitrogen-containing atmosphere (inclusive of natural atmosphere) thereby enabling the metallic aluminum powder incorporated in the alumina to be converted into aluminum nitride and effecting incorporation of the aluminum nitride in the fused alumina. When this reaction method is worked by using air as the nitrogen-containing atmosphere, the metallic aluminum powder incorporated in the alumina is nitrided and, at the same time, a considerable portion of the metallic aluminum powder is oxidized with the oxygen in the air. This method, therefore, has the disadvantage that the metallic aluminum powder used therein is not wholly utilized effectively and, as the result, the nitrogen content of the product cannot be easily standardized between different lots of production. For the purpose of eliminating the disadvantage described above and ensuring effective utilization of the metallic aluminum powder used in the production, therefore, it becomes necessary to keep the electric furnace containing the alumina wholly in a tightly closed state and carry out the work of electric fusion with the interior of the furnace filled completely with an atmosphere solely of nitrogen gas. As the result, the operation becomes complicated and the cost of equipment becomes high.